1. Field of the Invention
The present invention relates to a thin film transistor (TFT), a method of forming the same, and a flat panel display device having the same, and more particularly, to a TFT capable of dispersing an electric current by infiltrating a metal into source and drain regions when the TFT operates, whereby charge mobility is improved, and damage of a drain region caused by the excessive current density is prevented, leading to the long lifespan and excellent performance, a method of forming the same, and a flat panel display device having the same.
2. Discussion of Related Art
A thin film transistor (TFT), which is a type of a field effect transistor (FET), is commonly not only applied to a static random access memory (SRAM) or read only memory (ROM) but also used as a pixel switching element of an active matrix flat panel display device. For example, a TFT is used as a pixel switching element or a current driving element of a liquid crystal display (LCD) device or an organic electro-luminescence display device. A TFT used as such a switching element serves to isolate individual pixels from electrical influence of neighboring pixels and to transmit an electrical signal to the pixels.
A semiconductor employed in a TFT is usually amorphous silicon (amorphous Si) or polycrystalline silicon (poly-Si). When current driving ability of a TFT or mobility of a charge carrier is considered, poly-Si is more suitable for a semiconductor used in a TFT. On the other hand, when a processing temperature or an available substrate is considered, amorphous Si is more suitable for a semiconductor used in a TFT, and thus semiconductors used in TFTs have mostly been formed of amorphous Si so far.
However, charge mobility in a TFT which uses amorphous Si is only about 0.5 cm2/Vs. Therefore, in an LCD which uses a TFT having a channel formed of amorphous Si as a switching element, it is difficult to increase the operating speed. As a result, research on forming a channel of a TFT formed of a material having a higher charge mobility than amorphous Si or changing a structure of a TFT to increase charge mobility has been conducted.
FIG. 1 is a cross-sectional view illustrating a problem occurring in a conventional TFT.
As shown in FIG. 1, the conventional TFT includes a buffer layer 2 disposed on a substrate 1. A semiconductor pattern 3 is disposed on the buffer layer 2, and an insulating layer 4 and a gate electrode 5 are disposed sequentially on the semiconductor pattern 3. The semiconductor pattern 3 below the gate electrode 5 functions as a channel area. A source region 6 and a drain region 7 are disposed in parallel to the semiconductor pattern 3.
If the TFT is used as a driving element of an LCD as described above, the TFT has a charge moving path CP1 which is wider in a direction of the drain region 7 from the channel region 3. Charges are concentrated in a local area A while moving to the drain region 7, and thus the extent to which charges are dispersed is increased in the local area A. As a result, there occurs a problem in that the charge mobility is decreased. Also, the charge moving path CP1 becomes almost perpendicular in the drain region 7, and thus stress is continuously applied to the local area A. Consequently, as a time goes by, driving characteristics of the TFT deteriorate, leading to a short lifespan.